Transformed cells are known to be more metabolically active than non-transformed cell. This increased metabolic activity is the result of the deregulated growth controls in the transformed cells. Increased metabolic activity has many effects, including, but not limited to, increased reactive oxygen species (ROS) production. If not countered by cellular anti-oxidant defenses, increased production of ROS species can lead to cellular damage, increased mutation rates and even cell death, such as via activation of the apoptotic pathway. Therefore, these transformed cells are dependent on cellular anti-oxidant defenses for survival. As a result of this dependence, if such transformed cells could be deprived of the chemical building blocks required to sustain the anti-oxidant defenses, the growth and/or viability of the transformed cells could be decreased.
Glial-derived tumors (i.e. gliomas) are transformed cells that display increased metabolic activity as a result of the transformation process. Gliomas comprise a diverse group of neoplasms that differ in their morphology, their CNS location, their degree of invasiveness, their tendency for progression, and their growth characteristics. Neoplastic transformation can occur in all glial cell types, thereby producing a large range ofpathological and morphological variants. Most primary brain tumors derived from glial cells have lost growth control regulation, giving rise to astrocytomas, glioblastomas, or oligodendrocytomas. High-grade gliomas account for 30% of primary brain tumors in adults, and are the second most common cause of cancer death in children under 15 years of age (13, 14). High-grade gliomas are divided by grade into two categories: anaplastic astrocytomas (WHO Grade III) and glioblastoma multiforme (GBM; WHO Grade IV) (15). There are also two other histopathologically classified grades of brain tumors, namely, Grades I and II. Increasing grades represent increasing malignancy and decreasing differentiation, which is associated with increased mitotic activity and enhanced cell migration (16, 17).
As a result of their increased metabolic activity, glioma cells have been shown to produce large quantities of ROS. In response to this increased production of ROS, glioma cells have been shown to produce increased levels of antioxidants, such as glutathione. Cystine is an essential precursor in the synthesis of glutathione, an important intracellular antioxidant responsible for scavenging ROS (1). It was believed cystine was transported into glioma cells via a variety of cellular pathways, including system Xc. System Xc is a Na+-independent glutamate transport system that has been functionally described for several decades (2). System Xc is highly expressed in glioma cells. System Xc is a heterodimeric protein complex consisting of a catalytic light chain (xCT) that confers substrate specificity and a regulatory heavy chain (4F2hc) (3). Cloning studies have shown that xCT belongs to the family of 12-transmembrane domain amino-acid transporter proteins (3). xXT has been shown to exist in two splice variants, hxCTa and hxCTb, in gliomas. 4F2hc is a cell surface glycoprotein previously known as CD98 that is essential for membrane localization of the transporter (4). Only the heterodimeric protein complex functions as an amino-acid transporter.
Unlike glioma cells, system Xc is not implicated in cystine uptake in mature neurons or astrocytes (7, 8), which use Na+-dependent glutamate transporters for this purpose. Inhbition of cystine uptake by blocking system Xc, which would reduce cellular levels of glutathione and increase the susceptibility of glioma cells to ROS-mediated damage and cell death, would therefore be an effective treatment for gliomas. Importantly, such inhibition of system Xc would not negatively impact the function of non-transformed glial cells since they do not rely on system Xc for cystine uptake.
The prior art has not understood that glioma cells rely almost exclusively on system Xc for the uptake of cystine. As a result, methods for the treatment of glioma cells directed solely at inhibiting system Xc have not been described. Prior art methods utilized treatments that inhibited cystine uptake in non-transformed neural cells, often forcing the co-administration of compounds to address this issue.
The present disclosure describes generally methods for the treatment and prevention of disease states that require cystine for maintenance or progression of the disease state. In addition, methods for screening and identifying novel therapeutic agents useful in the treatment of such disease states are described. In one embodiment, the disease state is a cancer, such as, but not limited to, glioma. More specifically, the present disclosure describes methods for the treatment and prevention of glioma by inhibiting cystine uptake or decreasing intracellular cystine concentrations, thereby inhibiting the ability of glutathione to maintain ROS levels at conditions which are not harmful to the transformed glial cells.